Hydrocarbon development well cementing is a complex operation with multiple goals: mechanically secure the casing strings in the geologic formation, isolate a producing layer from adjacent layers, protect the strings against the corrosion due to the fluids contained in the layers crossed through. The cement sheaths therefore have to provide good mechanical strengths and low permeability to the fluids and to the gas contained in the formations drilled.
Under certain geothermal or hydrocarbon reservoir development conditions, it is essential to have cementing materials with both low densities and excellent physical properties (mechanical strength and permeability). These two conditions are difficult to combine with conventional cementing materials. It is well-known since Féret's research work that the mechanical strength varies conversely to the porosity. Féret notably showed that the compressive strength Rc was expressed as follows:
            R      c        ⁡          (      t      )        =            K      ⁡              (        t        )              ⁢                  (                  c                      c            +            e            +            v                          )            2      where c, e and v are the volumes of cement, water and air respectively, and K(t) a kinetic function.
In order to lighten cementing slurries, it is common practice to increase either the amount of water or the amount of air (using hollow balls or by entraining intentionally a large amount of air so as to form a cement foam). According to the above formula, these two means lead to a mechanical strength degradation and, simultaneously, to a great increase in the permeability of the hardened material.
When the formations drilled are fragile and unconsolidated, it is impossible to carry out operations with a dense cement slurry for fear of exceeding the fracture pressure of the formations. This problem is notably encountered when cementing the casings of offshore wells or wells drilled in mature fields.
To cement wells crossing fragile formations, i.e. with a low fracture gradient, it is well-known to significantly lighten the slurry by adding gas. This gas can be introduced by means of hollow ceramic or glass microspheres. This technique is notably described in documents U.S. Pat. No. 3,804,058 and U.S. Pat. No. 4,252,193. The gas can also be introduced into the slurry by creating a foam by means of foaming agents added to this slurry. This technique is notably described in documents U.S. Pat. No. 5,806,594 and U.S. Pat. No. 5,484,019.
Cements lightened by means of hollow balls have certain drawbacks. One drawback is the destruction of the balls under the effect of the hydrostatic pressure. This destruction translates into a density increase while pumping the slurry: the fracture pressure can thus be reached. Another drawback of hollow glass balls comes from the destruction, in the hardened cement, of the walls of the balls as a result of pozzolanic reactions. This destruction translates into an increase in the permeability of the cement matrix.
Common formulations of a foamed cement slurry for cementing wells comprise a proportion of water ranging between 40% and 60% by weight of cement. This high water amount, necessary to lower the cement slurry densities and to optimize the rheology, generates a high porosity which translates into poor properties of the cement sheath in terms of permeability, mechanical strength, cracking and durability.
The problem now consists in formulating a pumpable hydraulic binder foam (i.e. having a viscosity range compatible with the viscosities required for setting the slurry in the annulus) for cementing oil wells or other wells, with higher mechanical strengths and a lower permeability. The present invention therefore describes the way to formulate a low-density cementing material by introducing gas in form of bubbles that will be separated by a very compact cement matrix. In this case, although the material obtained is very porous, the permeability of this material remains very low because the invention described allows to obtain a foamed cement wherein the bubbles are not interconnected.
There are cement formulations with much better mechanical properties, as described for example in document EP-950,034. These formulations are based on maximization of the packing volume fraction by optimization of the grain size of the mineral particles. In fact, it has been known for a long time in the profession (see Féret's formula above) that the properties of cement materials are improved by increasing the compactness of the mixture (or, which comes to the same thing, by reducing the porosity). These materials can have compressive strengths above 100 MPa and gas permeabilities of the order of one nanoDarcy. It is well-known that the viscosity of suspensions increases exponentially with the volume fraction in solid particles: the significant increase in the cement slurry viscosity is very serious from an operational point of view because, in this case, the material can no longer be set in place by pumping. Now, in the invention described in document EP-950,034, optimization of the grain packing of the mixture of mineral powders achieved by properly selecting both the size of the mineral particles and their concentration allows to obtain slurries that are much more fluid than conventional cement slurries. Unlike conventional cements, the high-performance cementing materials described in document EP-950,034 can have a zero yield point. However, the densities of these high-performance cements as described in document EP-950,034 are above 1.9 g.cm−3 and they are therefore not suitable for cementing fragile and unconsolidated zones such as those encountered in deep-sea drilling or for cementing wells in depleted reservoirs. Recently, low-density formulations, typically ranging between 1.2 and 1.6 g.cm−3, were developed for cementing wells drilled in unconsolidated geologic layers. These cementing materials are high-performance materials to which hollow microspheres have been added. These materials thus have the same drawbacks as conventional cements containing hollow microspheres: microsphere crushing during pumping in the well, pozzolanic reaction between the portlandite and the silica contained in the microsphere walls. Furthermore, it is impossible to vary the density of the cement during cementing.
The present invention thus provides a cementing material formulation having simultaneously low densities and excellent physical properties, notably compressive strength and permeability. This combination of low densities and of improved physical properties in relation to the state of the art is achieved by foaming cements whose packing volume fraction is maximized by adjusting the proportions of the various grain size classes that make up the material.